Cells transformed by the avian MC29 virus and related viruses assume a characteristic microscopic appearance which differs from the changes seen in cells transformed by other viruses or other agents. We are interested in identifying transcriptional changes which occur specifically as a result of infection with these myc-containing viruses, which differ from general change which occur in all cells transformed to malignancy. Rat embryo cells were infected with SV40 virus, and clones of transformed cells were superinfected with murine retrovirus constructs containing the myc oncogene. Polyadenylated messenger RNA was isolated, and cDNA libraries were constructed and cloned in bacteriophage. The induction or repression of mRNAs by myc using these libraries and radiolabeled cDNAs is under investigation.